1. Field of the Invention
The present invention relates to a method injection molding ultra thin thermoplastic lenses. The lenses produced according to the invention have particular utility as carriers in the Transfer process as described in the patent application WO 03/004255.
2. Description of the Related Art
Lenses and other articles manufactured at optical quality, have exacting demands for mold replication, high optical transmission and impact resistance. In the 2002 Polycarbonates publication, Brunelle and Kailasam describe how polycarbonate is prepared by the transesterification of a bisphenol-A with a carbonate. U.S. Pat. No. 5,212,280 describes diphenols which are useful in the condensation method of producing polycarbonate in the presence of phosgene.
Injection molding of lenses requires edge gating so that the runner ends up remote from the lens surfaces. The paths from the gate to the edge points of the mold cavity are not symmetrical and therefore make it difficult to control the thermodynamics of the cooling melt flow. As lens cavities become thinner, straight injection molding techniques are unable to fill the mold without premature freeze-off. Accordingly, injection molding machines have been modified to enlarge the cavity during some phase of the injection cycle, in a so-called injection/compression process. An example of injection/compression utilizing auxiliary component hardware follows.
U.S. Pat. No. 5,972,252 uses auxiliary hydraulic cylinders to move the insert on the movable side of the mold. Toward the end of the injection cycle, the hydraulic cylinders advance the retracted insert toward the stationary side of the mold to compress the resin to its final thickness.
U.S. Pat. No. 6,616,868 overfills an enlarged cavity and during a compression step, the excess resin is forced back into the injection cylinder. In an alternate embodiment, the pressure in the injection cylinder and the compression force are adjusted to deliver an amount of resin needed to achieve the target thickness.
U.S. Pat. No. 6,284,162 uses an auxiliary hydraulic cylinder to move one of the insert to create an enlarged cavity. After introduction of the resin, the hydraulic cylinder advances the insert to a hard stop point which corresponds the final part thickness. The reference attempts to overcome a series of problems associated with clamp-end injection/compression molding. Among the problems noted is the inability to mold thin centered lenses under the high clamp pressure and tendency for such systems to create flash and damage the equipment.
U.S. Pat. No. 5,415,817 employs a sleeve which extends past the insert to close the parting line while the insert remain retracted. When the insert begins advancing to compress the resin, excess molding material flows into an overflow well formed in the sleeve.
WO 00/71331 describes one embodiment which does not utilize auxiliary hardware, but employs a first low clamp force during initial filling of the mold. The injection screw then switches over to velocity-pressure mode and the second higher clamp force is exerted after which a coining operation occurs. In a second embodiment, the first clamp force brings an auxiliary third plate of a three plate mold to position corresponding to an enlarged cavity. The second clamp force than advances the third plate to compress the resin to its target thickness.
As can be seen from the above descriptions, auxiliary component injection/compression molding utilizes springs, cylinders or other mechanical means to exert a force on the insert and move it relative to its sleeve or its mold side. These systems typically exert a compressive force less than the clamp force, frequently in applications where it is believed that clamp force is excessive. However, they add cost and complexity to molding systems. Operators have to coordinate the clamp force in addition to the auxiliary component equipment and settings. Accordingly it would be desirable to injection mold high quality thin lenses with the simplicity of a clamp-end process.